1. Field of the Invention
The present invention relates generally to spinal fusion surgery apparatus. More specifically, the present invention relates to an interbody spinal fusion implant device adapted for insertion between opposing vertebrae to maintain separation thereof during the fusion process.
2. Description of the Prior Art
Spinal fusion surgery is often undertaken in treatment of pain associated with degenerative spinal disc disease and segmental instability of the spine. An intervertebral disc, which separates adjacent vertebrae, is comprised of a gel-filled nucleus encased by 18 rings of ligaments forming an exterior annulus. The most common type of disc failure occurs when the exterior annulus of the disc ruptures. A degenerated disc can result in spinal instability manifested as excessive movement of adjacent vertebrae which causes irritation of nerves resulting in back and leg pain. The object of spinal fusion surgery is to meld separate adjacent vertebrae into a single rigid bony mass in order to restore stability to the spine.
Spinal fusion surgical procedures generally include instrumented and non-instrumented fusion procedures. Non-instrumented fusion includes placing fusion promoting material between adjacent vertebrae to form shims in the corresponding intervertebral spaces to facilitate a calcification response by the body which effectively fuses the bony vertebral bodies together. The fusion promoting material may include bone plugs and bone chips harvested from the patient, and other osteo-inductive/conductive materials. Non-instrumented fusion techniques are implemented without installing any hardware to the spine to provide added rigidity, or intervertebral spacing.
Instrumented fusion procedures include the implanting of hardware such as intervertebral spacers, metal plates, pedicle screws, and rods for fastening the vertebrae together to provide rigidity and intervertebral spacing. Instrumented fusion procedures traditionally involve using a posterior approach that requires forming a large incision in the patients back, and stripping large muscles of the lower back away from the spine to allow access to the degenerated disc. A frequently used variation on posterior fusion is called a 360 degree procedure in which the spine is initially accessed using an anterior approach through a smaller incision in the patients abdomen, and subsequently accessed via another small incision in the patients back to insert instrumentation.
A stand-alone implant is a type of hardware commonly used in accordance with instrumented spinal fusion surgery wherein no additional instrumentation, such as hook and rod systems, is used to facilitate the spinal fusion. Stand-alone implants are inserted into a bore formed between adjacent vertebrae. A commonly used example of a stand-alone spinal fusion implant device is an interbody fusion cage.
Conventional interbody fusion cages typically include a hollow structure having an internal chamber for receiving fusion promoting material, the internal chamber being surrounded by external walls. The structure includes a plurality of openings formed in the exterior walls, the openings providing communication of the fusion promoting material between the internal chamber and the surfaces of the vertebrae between which the fusion cage is implanted. Communication of the fusion promoting material with the surfaces of the vertebrae induces calcification of the adjacent vertebrae for fusing the vertebrae together.
One of the most important advantages associated with the use of interbody fusion cages is the ability to perform spinal fusion surgery in a less traumatic way for the patient. Surgical techniques for implanting interbody fusion cages generally include the steps of: forming a bore between adjacent vertebrae; inserting a fusion cage into the bore; and inserting the fusion promoting material into the internal cavity. Currently, the most common surgical technique for implanting interbody fusion cages includes implanting two fusion cage devices at each level of the spine to provide balanced intervertebral spacing and support. In addition to the options of performing an implant procedure using traditional anterior and posterior techniques, interbody fusion cages allow surgeons the option of performing an implant procedure using laparoscopic techniques. Kuslich (U.S. Pat. No. 5,700,291, issued Dec. 23, 1997) describes one example of a laparoscopic spinal stabilization method wherein a cylindrical implant is passed through a cannula and inserted into a bore formed between opposing vertebrae.
Michelson (U.S. Pat. No. 5,593,409, issued Jan. 14, 1997) discloses an example of an interbody fusion cage device including a partially cylindrical structure formed by a wall surrounding an internal chamber for receiving the fusion promoting material. The wall has a plurality of openings passing therethrough providing communication between the internal chamber and surfaces of the vertebrae between which the device is implanted. The exterior surface of the device includes numerous ridges and channels for reducing the probability of dislodgment of the implant after insertion.
Kuslich et. al. (U.S. Pat. No. 5,489,307, issued Feb. 6, 1996) describes another cylindrical implant device which provides external threads facilitating screw-in fixing of the device between upper and lower vertebrae to provide support and correct vertical dimensions in the intervertebral space. Kuslich et. al. also describes tools and methods for insertion of implants. An insertion tool described by Kuslich et. al. includes a drill tube having serrated axially projecting teeth on the distal end. The serrated teeth of the tool dig into the bony exterior of the adjacent vertebrae, holding the tool secure and providing a guide for a reaming tool. The teeth of the tool do not cut deep, but merely provide a secure anchor for the tool on the exterior surface of the vertebrae. The ossicified bone within the cylinders is not exposed to the same compressing load from the weight of the open body, and therefore becomes very porous - no longer providing cephalo-caudad support and creating a weak interface with the bony matter surrounding the cage.
One common problem associated with conventional interbody fusion cages is that, despite being integrally formed using titanium, they are breakable. Conventional interbody fusion cages are prone to fracture due to vertical forces brought to bear across the hollow cage structure which is squeezed between the opposing vertebrae. A fractured cage no longer provides support and may be prone to movement, or migration.
Another problem associated with conventional interbody fusion cages is migration from the original implanted position. Conventional interbody fusion cage devices are rigid compared with the softer bone material of the vertebral bodies. This relative rigidity of the device may lead to disassociation from the surrounding fusion mass as a result of an inability to flex with the natural movement of the vertebrae and their interstices. Migration of the cage typically arises in response to mechanical manipulation occurring during normal physical activity wherein the more rigid titanium cage breaks free of it""s bond with the surrounding bony material which is more flexible and compliant. Another factor which may contribute to migration is osteoporosis which is progressive and which leads to decreasing rigidity and strength of bones. This increases the potential for migration over time.
In an attempt to alleviate migration problems, some prior art devices include numerous ridges and channels formed on an exterior surface of the device for reducing the probability of dislodgment of the implant after insertion. Other devices include threaded walls which screw into bores formed between the vertebrae. However, migration remains a problem even in these types of devices.
Still another problem associated with conventional interbody fusion cages is that the wall surrounding the internal chamber substantially isolates the fusion promoting material inside the chamber from the normal stresses of weight bearing and physical activity. This is problematic because bone becomes weaker in the absence of loading. Calcium will be reabsorbed from the bone if it isn""t loaded, and the bone will weaken. According to accepted bio-mechanical orthopedic theory, bone tissue which is not exposed to sufficient stress or which is not used effectively, does not harden to its full potential, and is prone to re-absorption. Due to the enclosed design of the cage, most of the fusion promoting material is effectively isolated from load bearing duty due to its enclosure within the walls of the internal chamber. Being thus insulated, the fusion promoting material is prevented from developing strength and hardness characteristics equal to that of the surrounding fusion mass.
Another problem associated with conventional interbody fusion cages is that they are difficult to retrieve, or remove. If a fusion cage requires removal, the calcification around the cage and the ingrowth of bony material makes retrieval very difficult.
What is needed is an interbody spinal fusion implant device which provides enhanced vertical (cephalo-caudad) and radial support between the vertebrae, increased strength characteristics, and increased resistance to compression fracture.
What is also needed is an interbody spinal fusion implant device capable of receiving fusion promoting material while installed between adjacent vertebrae, the structure providing an increased area of direct exposure of the fusion promoting material to the opposing vertebrae thereby subjecting the fusion material to increased load bearing duty and compression to promote strengthening of the fused material, and to achieve enhanced fusion of the vertebrae.
Further needed is an interbody spinal fusion implant device which is not prone to migration from its original implanted position.
It is therefore an object of the present invention to provide an improved interbody spinal fusion implant device adapted to be installed by surgical techniques including the laparoscopic approach, the anterior approach, or the posterior approach.
It is another object of the present invention to provide an interbody spinal fusion implant device which provides enhanced vertical (cephalo-caudad) and radial support between opposing vertebrae, increased strength characteristics, and increased resistance to compression fracture.
Yet another object of the present invention is to provide an interbody spinal fusion implant device having a structural framework capable of receiving fusion promoting material while installed between adjacent vertebrae, the framework providing an increased area of direct exposure of the fusion promoting material to the vertebrae thereby subjecting the material to increased compression to promote strengthening of the fused material, and to achieve enhanced fusion of the vertebrae.
A still further object of the present invention is to provide a spinal fusion implant device which is not prone to migration.
Briefly, a presently preferred embodiment of the present invention provides a spinal fusion implant device for insertion into a generally cylindrical bore formed between adjacent vertebrae, the device including: a shaft having a longitudinal central axis; and a plurality of projections extending radially from the shaft and terminating at a cylindrical locus concentric with the shaft. Interstitial spaces formed between the projections provide a receptacle for fusion promoting material subsequently injected thereinto. In one embodiment, each of the projections is shaped and oriented to form a thread like segment for engaging the internal surface of the cylindrical bore to advance the device axially into the bore upon rotation of the device about the longitudinal central axis of the shaft.
In one embodiment, each of the projections includes an elongated member terminating in a surface at its distal end including at least one rib adapted to form the thread like segment for engaging the internal surface of the bore. The elongated member further includes a bulbous head forming the surface at the distal end of the elongated member.
An important advantage of the present invention is that the projections provide enhanced vertical (cephalo-caudad) and radial support between the vertebrae.
Another advantage of the present invention is that the interstitial spaces formed between the projections provide a large amount of mechanical purchase on the surrounding fusion material and bone. This mechanical purchase inhibits migration of the device from its original implanted position.
An additional advantage of the present invention is that fusion promoting material inserted between the plurality of projections and the shaft is provided with a large area of direct contact with the adjacent vertebrae thereby subjecting the fusion promoting material to the normal stress of weight bearing thereby promoting strengthening of fused material, and achieving enhanced fusion of the vertebrae. The increased bone compression provides increased longevity of the fusion.
The foregoing and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment which makes reference to the several figures of the drawing.